This invention relates generally to automatic control systems. More specifically, it relates to electric control systems wherein the control signals utilized are combined functions of a proportional signal and the integral and derivative thereof.
Automatic control systems are known in the art. Such systems receive electrical signals representative of a condition of an element to be controlled and compare that signal with a reference to produce an error signal. A particular application of such control systems has been as governors controlling the speed (RPM) of internal combustion engines.
In many applications it is desired to control engine RPM to maintain constant speed in spite of variations of engine load. It is desirable to provide a system wherein the transient response, that is, the response time of the engine and the control system, due to a change in operating conditions, is minimized. This avoids over speed, hunting and instability conditions potentially damaging to the system.
An automatic control system having the characteristics desired for the aforementioned uses is disclosed in co-pending U.S. Pat. application Ser. No. 538,133 filed Jan. 2, 1975, now U.S. Pat. No. 4,070,609 and assigned to the present assignee. In that application, which is hereby incorporated by reference, an automatic control system is described in which engine speed is sensed by a magnetic pickup coil. After wave shaping and amplification the wave form is applied through an isolating switch arrangement to the input of an operational amplifier. This amplifier compares the average DC level against a reference to generate an error signal. The error signal is then differentiated and integrated with respect to time. A signal proportional to the error signal, its integral and its derivative are combined to produce a control signal for operating an actuator device coil. Various protective circuits are incorporated in the controller, such as, a current limiter and a fault detector for determining engine failure. By use of a feedback element associated with the actuator, accurate control over that device is obtained.
The gain of the automatic control system or, more specifically, the gain of the difference amplifier which generates the error signal must be selected according to the characteristics of the prime mover. These characteristics include system inertia, operating speed, rated horsepower, loading, and the like. If the gain is too high for a given application, the automatic control system will hunt, i.e., alternatively overshoot and undershoot its desired set point and in severe cases become increasingly unstable. In the case of too little gain from the error signal amplifier, the desired constant RPM cannot be maintained with sufficient accuracy. That is, the transient response is too long.
A special problem is produced where the characteristics of the prime mover change significantly during a period of operation. Under those circumstances, it is necessary to alter the gain of the difference amplifier for each set of operating conditions if maximum performance is to be obtained.
It is accordingly an object of the invention to provide a novel automatic control system which is sensitive to variations in a controlled element to maintain a desired condition.
Another object of the invention is to provide a control system which is responsive to changes in the external operating characteristics of the controlled prime mover for altering the system gain.
A further object of the invention is to provide a control system in which the system gain is switchable according to the load conditions of the prime mover being regulated.
Another object of the invention is to provide a control system which is particularly effective for controlling the speed of internal combustion engines so as to conserve fuel, accurately maintain a desired speed, and prevent engine run away.
Other objects and advantages of the invention will be apparent from the remaining portion of the specification.